The state Court of Appeals on Thursday reinstated a lawsuit against the online firearms brokerage where a man obtained the gun used to kill three people and himself at a Brookfield spa in 2012.

It is the first court in the nation to hold that a web-based gun marketplace might be held liable for negligence in facilitating an unlawful weapons sale.

“There are far too many bad actors out there, particularly unlicensed online gun sellers, who are putting profits ahead of safety. This court decision puts sites like Armslist on notice that they can and will be held accountable,” said Kris Brown, co-president of the Brady Center to Prevent Gun Violence, which is part of the plaintiff's legal team.

Zina Daniel Haughton and two others were killed when her estranged husband, Radcliffe Haughton, shot up the Azana Spa where she worked. Four others were injured, and Radcliffe Haughton fatally shot himself. Zina Haughton's coworkers, Cary Robuck and Maelyn Lind, were killed.

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Police mark off the scene at Azana Salon & Spa in Brookfield after a man shot several people before taking his own life on Oct. 21, 2012.(Photo: Michael Sears/Milwaukee Journal Sentinel)

Yasmeen Daniel, personally and as administrator of her mother's estate, sued Armslist.com, on which Radcliffe Haughton found someone to sell him a gun and ammunition while he was prohibited by a domestic violence injunction from having firearms.

Milwaukee County Circuit Judge Glenn Yamahiro had dismissed the suit in 2016, citing the immunity granted to "interactive computer services" under the federal Communications Decency Act. He found that Armslist only displays content created by third parties and couldn't be liable for being just the publisher of others' content on its site.

Radcliffe Haughton(Photo: Brookfield Police Department)

But the Court of Appeals agreed with Daniel that her suit does not seek liability for those protected actions, but for using a "website design feature to facilitate illegal firearms purchases."

Many other online marketplaces, such as eBay, Craigslist and Amazon do not allow posts seeking to buy and sell firearms.

"To repeat, Daniel argues, and the complaint alleges, that Armslist is liable for designing and operating its website in a way that encouraged prohibited sales, which she contends was a substantial factor in causing the shootings," Judge Brian Blanchard wrote for the court.

"It may be fair to characterize all of the operational and design features alleged by Daniel to be in some sense 'content-based.' However, in this respect, the content is not 'information provided by another information content provider.'

"Rather, it is content created by Armslist, and there is no language in the Act immunizing Armslist from liability based on content that it creates."

The court also reversed Yamahiro's decision to dismiss Daniel's claim of negligence per se. The case will now return to Milwaukee County circuit court.

Other groups had filed friend-of-the-court briefs in the case, including the National Coalition Against Domestic Violence and Everytown for Gun Safety.